1. Field of the Invention
The present invention relates to an air-fuel ratio controlling system for a fuel-injection type internal combustion engine and, more particularly, to an air-fuel ratio controlling system of the type that includes an air valve disposed in an intake duct of the engine upstream of a throttle valve to cooperate with the throttle valve to define an air pressure chamber, a fuel-metering device disposed in a fuel circuit and cooperative with the air valve normally to maintain the air-fuel ratio at a substantially constant value, and an O.sub.2 sensor provided to detect the oxygen content of engine exhaust gases and to emit an output signal which is utilized to adjust the air-fuel ratio.
2. Description of the Prior Art
There has been known an air-fuel ratio controlling system for an internal combustion engine of the type that has an intake duct with a throttle valve disposed therein. The system has an air valve disposed in the intake upstream of the throttle valve to cooperate therewith to define an air pressure chamber therebetween; an air valve controlling means responsive to variation in the air pressure in the air pressure chamber normally to maintain a substantially constant air pressure difference across the air valve; a fuel circuit having at its downstream end a fuel discharge port open to said intake duct; a fuel metering means defining a fuel-metering variable orifice disposed in the fuel circuit and means for maintaining a substantially constant fuel pressure difference across the fuel-metering variable orifice; the fuel-metering orifice defining means being operatively associated with the air valve so that the fuel-flowing area of the fuel-metering orifice is varied in proportion to the variation in the air-flowing opening area defined between the air valve and the intake duct; the air valve controlling means and the fuel metering means being cooperative to control the air-fuel ratio of an air-fuel mixture to be fed into the engine such that the air-fuel ratio is normally kept substantially constant; an O.sub.2 for detecting the oxygen content of the engine exhaust gases; and means responsive to signal from the O.sub.2 sensor to adjust the air-fuel ratio.
An air-fuel ratio controlling system of the type briefly discussed above is disclosed in Japanese Laid-Open Publication (Pre-Examination Publication) No. 48-83220 published on Nov. 6, 1973.
In the air-fuel controlling system disclosed in the Japanese publication referred to above, the O.sub.2 sensor responsive means comprises an electro-magnetically operated valve disposed in a fuel return line of the system to control the fuel pressure in the fuel circuit downstream of the fuel-metering variable orifice thereby to vary the fuel pressure difference across the fuel-metering orifice so that the air-fuel ratio of the mixture to be fed into the engine is adjusted in accordance with the output of the O.sub.2 sensor.